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A New Piece of the Global Methane Puzzle

30 March 2017

The first cells on Earth may have feasted on the byproducts of serpentinization, reactions between water and mantle rocks that, in the presence of carbon, may generate large amounts of methane and other organic compounds. Serpentinization reactions at shallow depths are the subject of intense research interest due to their possible contributions to life’s origins, but also because they play an important role in Earth’s carbon cycle.

Now, a new study suggests that these same chemical reactions occur not just at the surface, but also deep beneath Earth’s crust, under conditions of high temperature and pressure. DCO Deep Energy Community members Alberto Vitale Brovarone (Institut de Minéralogie, de Physique des Matériaux et de Cosmochimie, France) Isabelle Martinez (Institut de Physique du Globe de Paris, France), and colleagues studied rock samples from the Lanzo Massif in the Western Alps. The analysis shows that serpentinization reactions deep down can generate methane at rates that are comparable to shallow serpentinization environments. The researchers report their findings in a new paper in Nature Communications [1].

“For the first time, our samples demonstrated that a huge amount of methane can be produced at these depths by processes related to serpentinization,” said Vitale Brovarone. “This has implications not only for Earth, but also for the exploration of other planets, for example, Mars.”

The researchers chose the Lanzo Massif, located in the Western Italian Alps, near Vitale Brovarone’s hometown, as their study site. In the Jurassic Period, the massif made up part of the mantle rocks flooring the Tethys Ocean, between the Europe and Africa paleocontinents, but moved deeper during subduction, when pushed by the successive convergence of the two continents. The collision of plates that created the Alps later exhumed the rocks. Interactions with fluids at the seafloor and while in the subduction zone created different stages of serpentinization within the massif itself.

The researchers specifically sought out rocks containing carbonate, called ophicarbonates, which had undergone metamorphism in the massif. They then looked for evidence of deep methane that formed abiotically, from physical rather than biological processes.

Using a combination of field and microscopic examination, chemical analysis of mineral compositions, and stable isotope analysis, the researchers show that serpentinization reactions can occur at high pressure and temperature in subducted rocks as deep as 40 kilometers below the surface. The hydrogen these reactions generate reduces the carbonate in the rocks to form large quantities of methane, some of which precipitates out as graphitic carbon. This graphitic carbon can then reconvert to abiotic methane through successive interactions with the deep hydrogen.

Stable isotope analyses show that the graphitic carbon that precipitates from the methane has signatures comparable to diamonds, which suggests that similar processes may be at the origin of diamond formation at greater depths.

The researchers also used thermodynamic modeling and mass balance to estimate how much abiotic methane the process produced and for how long. They calculate that this process released about 350 kilograms of methane per cubic meter of rock and that the global methane release from such a process may amount to more than 4 megatons each year, which is comparable to or more than the methane released by serpentinization processes occurring at the seafloor. The collected data suggest that the Lanzo Massif has released deep methane for at least one million years.

In future work, Vitale Brovarone and his colleagues plan to identify mineral markers associated with deep abiotic methane generation, so that they can locate the process in other locations on Earth and beyond.

The findings suggest that researchers may need to add a new line item to the global carbon budget for abiotic methanogenesis occurring in subduction zones.

“The mechanisms that may allow this deep methane to come to the surface and seed the biosphere are not known, but this is a unique reservoir and cannot be excluded from the big picture,” concludes Vitale Brovarone.

The Deep Carbon Observatory (DCO) is a global community of multi-disciplinary scientists unlocking the inner secrets of Earth through investigations into life, energy, and the fundamentally unique chemistry of carbon.